Seriously, though, they're adding features. Vista won't run like XP did on the same hardware, XP won't run the same as 2k, OSX won't run the same as OS9.
I don't know much about Linux, but would it be possible to build your own from source, picking and choosing features to get maximum performance? It looks like they've identified potential problem areas.
If you've got the requisite knowledge, you can do just about anything. Some people made Fedora running on an Eee achieve a 5-second boot. While booting isn't nearly the same as general "speed" in a system, it just goes to show how much can be cut out and done synchronously to help things along.
In a desktop environment, your main factors for slowdown are going to be more along the lines of your window managers and your eye candy. Things like E17 are window managers that claim to have blended the best of both worlds, and have a fast, small-footprint, nice-looking GUI, but I've yet to try them out, as I haven't been able to get Ubuntu fully loaded on my new laptop yet.
My point is just that there's plenty to be done if you really have the mindset and knowledge to go get it done. It's just that, much like Windows, Ubuntu is trying to appeal to and support a LARGE range of hardware, and that's when you get into the arena of including things that not everybody needs. Optimizations are wonderful things, but the best ones only come when you code them for specific hardware. (See: Apple.)